Selective circuits



y 1936- H. A. WHEELER 2,048,527

E v I SELECTIVE CIRCUITS Filed Feb. 1, 1932 5 Sheets-Sheet 1 5 F2: FrINVENTOR HAROLD A. WHEELER BY PM, 15m,

ATTORNEYS H. A. WHEELER SELECTIVE CIRCUITS July 21, 1936.

Filed Feb. 1,

3 Sheets-Sheet 2 HAROLD A. WH EELER ATTORN EY$ July 21, 1936. W E2,048,527

SELECTIVE CIRCUITS Filed Feb. 1 1932 3 sheets sheet s Q Q Q Q Q Q 3 /rcI I29, 70' 5 Q Q j a E k INVENTOR HAROLD A. WHEELER Primarily -21. 1936s "PATE T OFFICE snmc'nvn cracurrs Harold A. Wheeler, Great Neck, N. Y.,ssslgnor to Haseltine Corporation Application February 1, 1932, SerialNo. 590.113

19 Claims. (Cl. 250-20) The present invention relates to improvements inselective electrical circuits, and more particularly to selective.circuits for preventing the reception of image frequencies" in theoperation of superheterodyne radio receivers.

A superheterodyne type ofradio receiver has an inherent defect in thatit is responsiveto signals of two frequencies each separated from theoscillator frequency by an 'amount which is equal to the intermediatefrequency. Thus, when such a receiver is tuned to receive a desiredsignal, in the presence of undesired signals differing therefrom by anamount which is twice the intermediate frequency, such undesired signalsare received, converted into the intermediate frequency, and interferewith the desired signal, which has also been converted into theintermediate frequency. The frequency of such interfering signals towhich the superheterodyne receiver is responsive is known as the imagefrequency. Tuned circuits must be relied upon to suppress signals ofthis frequency and to prevent their being converted into theintermediate frequency.

It is the object of the present invention to overcome the above-noteddisadvantage of the. superheterodyne radio receiver and provide a newand improved selective circuit for preventing the reception of. signalsof the image freao quency.

It is a further object of this invention to provide an image rejectioncircuit in which the response to the desired signal will not be in anyway diminished, although the tunable circuit has 5 a very highselectivity against image interference, independent of the frequency towhich the receiver is timed.

For accomplishing the objects of the present invention, a selectivenetwork is provided for 49 selectively transferring desired signalfrequency currents of any frequency in a band of frequen-- cies. Thisnetwork is connected between the exciting circuit, or antenna, of areceiver and a responsive device, or the receiver itself. The 45selective network comprises a broadly resonant circuit and a selectivecircuit. The resonant circuit which may be the antenna circuit isbroadly tuned to all frequencies within the band of frequencies forattenuating all signals of any 50 frequency outside of said band. Theselective circuit is connected between said broadly resonant circuit andsaid receiver, and is for suppressing all image frequency currents whichare admitted by the antenna circuit and which fall 55 within said band.

The selective circuit comprises two signal transfer means each coupledto the broadly resonant circuit and to the receiver. The first transfermeans has a transfer ratio which varies greatly with frequency and has amaximum 5 transfer at the frequency of the signal, and thus selectivelytransfers a signal current of any frequency within a band offrequencies. However, this transfer means incidentally transferscurrents of the undesired image frequency though 10 to a lesser degree.The second, or auxiliary transfer means, has a. transfer ratio whichdoes not vary greatly with frequency, and thus nonselectively transferscurrents of the entire band of frequencies. These two transfer means are15 adjusted to respond equally and oppositely to signals of the imagefrequency. Thus, the relative intensity of the image frequency currentsas first and secondly transferred is so adjusted that .the currents maybe balanced against each other 20 and thereby image frequency currentsfalling within said band be completely suppressed. The first-mentionedtransfer means builds up by resonance the selectively transferredcurrents and thus effects a much greater transfer. at the carrierfrequency of the signal than the non-selective transfer of the carrierfrequency by the second transfer means. The second transfer meanstherefore has but little effect upon the transfer of the desiredfrequencies. Also the voltages transferred by'these two transfer meansare not in opposition at the desired signal frequency, as will beexplained later.

The amount of non-selective transference is adjusted to equal the amountof image frequency selective transference throughout the band offrequencies. Thus, the circuits are so designed 'and proportioned thatthe frequency of complete suppression varies automatically andsimultaneously with the adjustment of the selective 40 transference atthe resonant frequency of the tunable circuit, and the frequency ofcomplete suppression is maintained substantially uniformly spaced fromthe resonant frequency of the the resonance of the entire input circuitso that the transfer of the image frequency through the untuned circuitwill be correspondingly varied.

Whereas this invention has particular application in connection withimage frequency suppression, as discussed above, it is to be understoodthat it also has considerable utility in general radio reception forselectively transferring currents of a desired signal frequency andpreventing interference of signals of any frequency whatever.Furthermore, the principles of this invention may be applied to each ofa number of resonant circuits in one system, and thereby secureproportionately greater selectivity.

Attention is now invited to the drawings, in which:

Fig. 1 is a schematic diagram illustrating one arrangement of the twotransfer means;

Fig. 2 is a similar diagram illustrating an alternative arrangement ofthe two transfer means;

Fig. 3 is a diagram showing a curve illustrating the selective'action ofthe circuits of Figs. 1 and 2;

Fig. 4 is a diagram showing a radio antenna circuit embodying theprinciples illustrated in Fig. 1;

Fig. 5 is a diagramv showing a modification of the circuit of Fig. 4;

Fig. 6 is a circuit diagram showing a second modification of the circuitof Fig. 4;

Figs. 7 and 7A are diagrams showing a set of curves illustrating theselective action of the circuit of Fig. 6.

In Fig. 1, to which attention is now invited, there is shown a selectivecoupling system comprising exciting circuit 1 including a source-0f.alternating current I0, and the two impedances Z1 and Z1, the latter ofwhich form a path between the input terminals l2 and it. There is alsoshown a work circuit 8 exemplified by the grid-cathode circuit of thevacuum tube It. The exciting circuit is coupled to the responsive deviceby two transfer means. The first transfer means includes the circuit 9composed of impedonce Z1, inductance L and condenser C. The sec- 0ndtransfer means includes impedance Zz. There is thus provided an outputpath including the condenser C and the impedance Z1, connected in seriesbetween the output terminals I3 and I4.

The first transfer means 9 is sharply resonant at or near a frequencywhich will be called the signal frequency. This sharp resonance iseffectively maintained by making Z1 sufllciently small or by making theimpedance of the source l0 sufficiently large so that the excitingcircuit does not have a controlling effect upon the resonance of thefirst transfer means 9. For the same purpose, the impedance of theresponsive device It is made sufiiciently large. The current I from thesource In flows through Z1, builds up a circulating current in the firsttransfer means 8, and thereby produces a voltage E1 across the condenserC. Y

The second transfer means Z1 is not sharply resonant at any operatingfrequency, and may be aperiodic. The current I, flowing throughimpedance'Zz, builds up a voltage E: across this impedance.

QJMQJD? v I 4 tem' iust ducribed may be reversed, retaining some of theadvantages of the system as shown inFig. 1; thatistosay. llandllmayeuledas input terminals and i2 and I! used as output terminals.

The transfer of the first means I will be denoted by the ratio 1321/1,and that of the second means, by the ratio Es/I, which latter is equalto Z1.

The combined resultant transfer will be denoted by Eo/I, in which theoutput voltage E0 is the vector sum of E1 and E2.

Fig. 2 is similar to Fig.1 except that L and C are interchanged. Theeffect of this change will be discussed below. In Fig. 2, like parts aredesignated by the same reference characters.

The operating characteristics of Fig. 1 will now be described, attentionbeing invited to Fig. 3.

Two frequencies will be considered as being derived from the source Ill.One is the desired signal frequency F1 and the other is the undesiredinterfering frequency F1, which is somewhat higher. or lower than F..The first transfer ratio E1/I is shown in Fig. 3 by the curve E1. Thiscurve shows a sharp maximum at F1, the resonant frequency of the firsttransfer means 9. The second transfer ratio Es/I is shown by the line E:indicating that this ratio does not have sharp resonance within theoperating frequency range.

The phase relationship of E1 and E1 will be explained. At F. theresonant circulating current in circuit 9 is in phase with Z1I, andtherefore E1 across C lags 90 behind Z1I. At frequencies lower than F1,the circulating current in circuit 9 leads nearly 90 ahead of Z1I and E1is nearly in phase with Z11. At frequencies higher than F1, thecirculating current lags nearly 90 behind'Z1I and E1 lags nearly 180behind Z11. E1 is always equal to M.

If Z1 and Z2, shown in Fig. 1, are of like character, the voltages E1and E1 differ in phase by 90 at F. and therefore their vector sum, asrepresented by the curve E0 of Fig. 3 at the point F1, is greater thanE1. At the same time, phase opposition between E1 and E: results atfrequencies higher than F1. At some frequency, F1 in Fig. 3, E1 and E:are equal and opposite, with the result that Eo/I is zero, as shown bycurve E0 in Fig. 3. Changing the value of Zn, but not its character,shifts the intersection of curves E1 and E2 so that any value of F1,higher than F., can be suppressed by using the proper value of Z2.Increasing the value of Z: brings the intersection nearer to F11. 0n theother hand, increasing the value of Z1 moves the intersection away fromF1.

If Z1 and Z: of Fig. 1 are made reactances of opposite kinds, the curvesof Fig. 3 will be reversed and the intersection of E1 and E1 will bebrought to a frequency lower than F1.

Interchanging L and C, as shown in Fig. 2, reverses the polarity of E:without substantially changing its value. Making Z1 and Z1 of likecharacter, in Fig. 2, brings the intersection of curves of E1 and E:below F1; while making Z1 and Z: reactances of opposite kinds, bringsthe intersection to a frequency higher than F1. Thus, interchanging Land C hasthe same effect as changing therelationship between Z1 and Z1,and making-both changes at once has substantially no eifectUDOII-thCr'ODBlfltlOll of the coupling system. Since any frequency F1can be suppressed in either Fig. l or Fig. 2 with the proper choice ofZ1 andZm'there is no essential difference between these alternativearrangements. If C is a variable tuning condenser, it is desirable toground the frame and moving parts. In this 76 case, the terminal ll ofthe condenser'C in Fig. 1. or the terminal I2 oi the condenser C in Fig.2, may be grounded. Also, the impedances Z1 and Z2 may be of anycharacter, such as mutual inductanees, thus producing a coupling circuithaving no common terminal l3 which is at once an input terminal and anoutput terminal.

When selective coupling systems such as those shown in Figs. 1 and 2 areconnected between the antenna and frequency changer of a superheterodynereceiver, F. is the signal frequency and'Fl the image frequency, F. andFl differing by twice the intermediate frequency. It is more common tomake F1 higher than Fa, as is shown in Fig. 3. The superheterodyneoscillator frequency F is midway between Fa and Ft, as indicated in Fig.3.

Fig. 4 is a circuit illustrating an adaptation of the principles of Fig.1 to a selective network or coupling system for coupling the antenna tothe first tube of a superheterodyne receiver. The coupling system istuned to the signal frequency F5 and has a great selectivity againstimage interference, as will be explained hereinafter.

In this figure, the antenna I6 is connected to the contact of a volumecontrol potentiometer l8, one part of which is' shunted by condenser H,the capacity of which is equal to the capacity of the average antennna.With this arrangement, the variation of the potentiometer tap does notmaterially affect-the tuning of the coupling system as a whole butprovides a means for varying the sensitivity of the combination. Theantenna current divides in potentiometer l8, one part flowing directlyto ground. The other part flows through the coil l9 and fixed condenserC1. The first transfer means is the closed circuit 9, which is composed,in this instance, of 01, L, C, of. which C1 corresponds to Z1 of Fig. 1.The circuit 9 is sharply tuned to resonance with the signal frequency F;by varying the capacity of the variable condenser C, and is connected tothe grid of the tube l5. Coil iii of the circuit 1' is coupled to thecoil 20 in the cathode lead of the tube I5. The mutual inductance Mabetween coils l9 and 20, which provides a fixed inductive couplingbetween the antenna circuit and the receiver, corresponds to Z; of Fig.1 and comprises the second transfer means. The condenser Cl. and theinductive coupling M2 are proportioned relative to each other so as todeliver equal and opposite voltages, at the image frequency, to theresponsive device or the receiver input and thus balance out imagefrequency signals which are within the band through which the resonantcircuit is tunable. It is to be noted that the image frequency signalswhich are thus balanced out are always separated from the desiredfrequency to which the resonant circuit is tuned by a constant frequencydifference.

Although many suitable circuit constants may be found for constructing acircuit in accordance with Fig. 4, it has been found that the followingare satisfactory:

The antenna l6 and the condenser I I may have a capacity of about 200micro-microfarads each, potentiometer i8 may have a resistance of about10,000 ohms, coil L may have an inductance of about 260 microhenrys,condenser C may have a capacity of 350 micro-microfarads (maximumsetting), and condenser C1 may have a capacity of about 3500micro-microfarads. Thus the fixed condenser C1 has a value of the orderof ten times the maximum value of the variable contual M2.

. ductance M1 in addition to C1.

denser C. The mutual inductance of coils I! and 20 may be about 4microhenrys.

The circuit of Fig.4, with the above values, is intended to tune toreceive a signal frequency of ,from 550 to 1500 kilocycles and isintended to constant separation bears a much'greater ratio to the lowersignal frequency. In order to compensate for the lower responsiveness ofthe tuned circuit to image frequencies, the ratio of the impedance of C1to the impedance of M2 is much greater at low frequencies. By thisimprovement, the image voltage at E1 and E2 very nearly counterbalance,regardless of the frequency of the signal being received.

Image transfer El/I increases proportionately to image frequency,because the decrease of impedance of C1 is more than offset by theincrease of image responsiveness of the tuned circuit 9. The imagetransfer Ez/I increases likewise, being equal to the inductive reactanceof the mu- The equality of the image voltages E1 and E2 is secured byselecting the correct values of the mutual M2, and phase opposition issecured by using the correct polarity of M2. Comparing Fig. l with Fig.4, Z1 is the negative reactance of C1, varying inversely with frequency,while Z2 is the negative reactance of the negative mutual inductance M2,varying'directly with frequency. The ratio of Z2 to Z1 varies as asquare of the image frequency.

When F1 exceeds Fs by a smaller constant difference, the ratio of Z2 toZ1 should vary less rapidly than the square of the image frequency, butmore rapidly than the first power. This is accomplished by the use ofthe improvements as embodied in Figs. 5 and 6, which will now bedescribed.

Fig. 5 shows an improved adaptation of the principles of Fig. 1 to asystem for coupling a radio antenna to the first tube of asuperheterodyne receiver. In this figure, like parts are designated bythe same reference characters. The circuit of Fig. 5 differs from Fig. 4in the following respects: First, the order of E1 and E2 in seriesbetween the grid and cathode of the tube I5 is interchanged in order topermit grounding of the cathode and of the frame and moving element ofthe tuning condenser C. Secondly, Z1 in Fig. 1 is replaced in Fig. 5 bythe mutual in- Thirdly, the antenna or primary circuit 1 of Fig. 5 isbroadly resonant within the tunable frequency range, resulting ingreater voltage amplification from the antenna to the grid of the tubel5, and in somewhat better image suppression. In Fig. 5, as in Fig. 4,Z: is replaced by the mutual inductance M2. In Fig. 5 the antenna isconnected to one side of a volume control rheostat 2| the variablecontact of which is grounded. The variable resistance or rheostat 2|serves also to produce a variable attenuation and broadens the resonanceof the primary circuit to include a band of frequencies. The antennacurrent divides, one part flowing through 2|, the other part through thecoils 22 and 24 and the condenser C1. The first transfer means is thesecondary circuit 9', which is composed of inductance L, variablecondenser C and the large fixed condenser C1, and the mutual inductanceM1 between coils 24 and L. The secondary circuit 9 is tuned to resonancewith the signal frequencyF; by variation of the capacity of condenserC.The impedance Z1 of Fig. 1 is replaced by the mutual inductance M1 andthe capacity of condenser C1. The large fixed condenser couples theprimary and secondary circuits only to a moderate degree and causes thetuning of the secondary circuit to be substantially independent of theprimary circuit. The impedance Z2 of Fig. 1 is replaced by the mutualinductance M1 between coils 22 and 23, which latter is connected inseries with the condenser C between the grid and cathode of the tube I5.

All the various parts of the circuit shown in Fig. 5 may have widelydifiering characteristics. The following values have been foundsatisfactory, and when used in the circuit shown, cause the circuit tooperate in accordance with the present invention:

The antenna 16 has a capacity of about 200 micro-microfarads, rheostator variable resistance 2| has a resistance of about 2.000 ohms, coils 22and. 23 have an inductance of about microhenrys each, the mutualcoupling M2 is about 4. microhenrys, inductance of L is about 260microhenrys, maximum capacity of C is about 350 micro-microfarads, andC1 has a capacity of about 3,500 micro-microfarads. The coil 24 shouldpreferably have a very small inductance, and the mutual coupling M1should be much less than M2, as will be explained more fullyhereinafter.

The circuit of Fig. 5, when constructed to embody the above values, isvery similar to that of Fig. 4, but is intended to work with a-kilocycle superheteroclyne amplifier. Therefore, the image frequency F1is 350 kilocycles higher than the signal frequency F5, and has a rangeof from 900 to 1850 kilocycles.

The antenna circuit effects some improvement in image suppression byattenuating all signals outside of the frequency band over which thereceiver is designed to operate, without regard to the independent meansfor balancing out of the voltages E1 and E2. The antenna circuit,including the antenna l6, coils 22 and 24, and condenser C1 in series,is resonant to a frequency near 1,000 kilocycles, which is about themiddle of the tuning range. The variable resistance 2| has a maximumvalue which is great enough to permit sensible resonance in the antennaprimary circuit, but which is small enough to broaden the resonance ofthe circuit to include all of the band, and the antenna resonance hasonly a negligible effect on the tuning of the coupling system as awhole. The variable resistance permits varying the sensitivity of thecombination. As has been mentioned, the inherent selectivity of thetuned circuit 9 against the image frequency currents is much less at thehigher frequencies. The broad resonance of the antenna circuit offersadditional selectivity against the image when F5 is in the middle orhigher part of the tuning range. As a result, the total inherentselectivity against the image frequency is given a higher average valueand is made uniform over the entire range. The signal voltageamplification from antenna to grid is also greatly improved as comparedwith Fig. 4, especially in the middle of the tuning range Up to thispoint, the effect of the grid to cathode capacitance, inherent in tubeII or in the connecting wires, has been neglected. The assumption wasmade that the responsive device represented by tube l5 had such highimpedance as not to have an important effect upon the tuning of thecircuit. This assumption is nearly met by Fig. 4, where only the directcapacitance between the grid and cathode is included between terminalsl3, l4, and not any capacitance to ground. In Fig. 5 this effect isgreater, including the capacitance to ground of coil 23 and grid wiringof tube l5. It is desirable to reduce this inherent capacitance to aslow a value as possible. Then its residual effect can be compensated forby making the ratio of Z: to Z1 abnormally high at the higherfrequencies. This result is automatically obtained in the followingtwo-point" adjustment.

A two-point adjustment of Fig. 5 is made possible by the condenser C1and the mutual inductance M associated with the first transfer means 9.By two-point is meant that at two points in different parts of thetuning range the frequency of maximum suppression will fall exactly atthe image frequency. At other points there may still be a slightdifference, so that the image does not suffer the maximum suppression,but this difference is negligible when the two-point adjustment isemployed.

With reference to Fig. 5, such an adjustment is made as follows: First,condenser C1 is made as small as permissible without too greatlyrestricting the tuning range of the circuit 9. This value of C1 has themajor effect in determining the coupling between the antenna circuit 1and the tuned circuit 9, and therefore the degree of voltageamplification from the antenna l6 to the grid of the tube 15. Arepresentative value of C1 is ten times the maximum value of C, althoughlower ratios may often be employed to advantage. Secondly, with themutual inductance M1 equal to zero, the system is tuned to a frequencyin the lower part of the tuning range, and M2 is adjusted to secure thegreatest suppression of the image frequency currents. Thirdly, thesystem is tuned to a frequency in the upper part of the tuning range,and the value and polarity of the mutual M1 are chosen to give thegreatest suppression of the image frequency currents at this portion ofthe tuning range. If great precision is desired, the second and thirdoperations may be repeated until no further improvement is possible. The

greatest suppression is thereby secured at two points, namely, the lowerand upper frequencies tuned in during the second and third operations,respectively. After making the two-point adjustment, nearly exact imagesuppression is secured over the entire tuning range. It has beenexplained that the character of Z1, and therefore the polarity of themutual M1, is determined, first, by the type of circuitemployed (Fig. 1or Fig. 2) and, secondly, by the location of the image frequency to besuppressed (above or below the signal frequency). Experience shows thatM1 has a relatively small value and that the correct value and polaritymay be best determined by trial. A

negative value of M1 is indicated in Fig. 5, which a is the correctpolarity for aiding phase relation between the mutual coupling componentand the capacitive coupling component, which couple 70 the square of thefrequency, but more rapidly 76 aoeacar' than the first power, which is ageneral requisite when the image frequency remains less than double thesignal frequency. Two guiding rules may be stated to govern the choiceof M1 in the cases under consideration, having image frequencies higherthan the signal frequencies: First, a smaller frequency diiferencebetween image and signal requires a larger negative value of the mutualM1. Secondly, a greater inherent direct capacitance across the terminalsl3" and it" requires a less negative value (or even a small positivevalue) of the mutual M1. The second rule, in extreme cases, amounts toan alteration of the general requisites just stated.

Fig. 6, to which attention is now invited, shows a preferred arrangementfor accomplishing the same purposes as the circuits shown in Figs. 4 and5, and for combining their respective advantages. In this figure,corresponding parts are designated by like reference characters. Fig. 6has the two-point adjustment and has the broadly resonant antennacircuit of Fig. 5, but has the ungrounded tube cathode of Fig. 4 inorder to minimize the direct capacitance across the terminals l3'" andH'". The circuit elements of Fig. 6 are like the corresponding elementsof Fig. 5. In Fig. 6, M: is the mutual inductance between the coils 22and 25, which latter coil has an inductance of about 10 microhenrys.Fig. 6, like Fig. 5, is intended to work with a 175-kilocyclesuperheterodyne amplifier so that the image frequency is 350 kilocycleshigher than the signal.

Experiments with Fig. 6 have shown that the two-point adjustmentprovides a, nearly complete image suppression over the entire tuningrange from 550 to 1500 kilocycles. This condition is graphicallyillustrated in the curves of Fig. 7 and 7A, to which attention is nowinvited. These curves show the transfer ratios plotted against frequencyas the system is tuned to 600, 1,000 and 1,400 kilocycles, respectively.In terms of Fig. 6, I is the exciting current flowing from the antennathrough coils 22 and 24 and condenser C1 and corresponds to I in Fig. 1.In Fig. 7 the curve E1 shows the ratio of El/I when a 1,000- kilocyclesignal is tuned in. This curve shows the inherent selectivity of thefirst transfer means tuned by condenser C. The voltage E: across thesecond transfer means is nearly imperceptible on the scale of Fig. '7.In order to better show the image suppression, the right-hand slope ofthe curve E1 is shown in Fig. 7A on a highly magnified scale ofordinates, but with the same frequency scale. It is seen that the ratioof El/I is only about one per cent as great at the image frequency of1,350 kilocycles as at the signal frequency of 1,000 kilocycles. In thisfigure, curve E: shows the ratio of Ea/I, which is the reactance of themutual inductance M2, having a value of 5.2 microhenrys in thisparticular case. As a result of the two-point adjustment, the curves E1and E2 intersect at the image frequency. Since E1 and E2 are of oppositepolarity, the resultant voltage E0, as indicated by the dotted line, iszero. The curve E represents the ratio Eo/I. In Figs. 7 and 7A thecurves E1 and E1" represent the corresponding curves for theGOO-kilocycle signal and 1,400-ki1ocycle signal, respectively. By theproper proportioning of the mutual M2, the slope ,of the curve E2, asshown in Fig. 7A, may be made so that it will intersect the curves E1and E1".at the proper point to permit the resultant curves E0 and E0" tobe zero.

Whereas the present invention has been described as being particularlyrelated to superheterodyne radio receivers, itistobcunderstood that theprinciples involved are equally applicable to general problems relatingto tuned oscillatory circuits. It is further to be understood-that theprinciples involved may be employed in succes- 5 sive tuning circuitsincluded in a single system and that proportionately greater selectivitymay be obtained thereby.

What is claimed is: a

1. An electrical coupling system comprising two input-terminals, twooutput terminals, a path between said input terminals, a path betweensaid output terminals, a circuit tunable to a desired signal frequency,said circuit including reactances of opposite kinds, a first impedanceincluding reactance common to both of said paths and said tuned circuit,and a second impedance common to both of said paths, the ratio of saidsecond to said first impedance having a variation more .rapid than thevariation in frequency as the frequency of the current impressed uponsaid circuit is varied.

. 2. An electrical coupling system comprising two input terminals, twooutput terminals, a path between said input terminals, a path betweensaid output terminals, a circuit tunable to a desired signal frequency,said circuit including reactances of opposite kinds, a first impedanceincluding reactance common to both of said paths and said tuned circuit,and a second impedance common to both of said paths, the ratio of saidsecond to said first impedance having a variation more rapid than thevariation in frequency as the frequency of the current impressed uponsaid circuit is varied and less rapid than the square of the frequency.

3. An electrical coupling system comprising two input terminals, twooutput terminals, a path between, said input terminals, a path betweensaid output terminals, a circuit tunable to a desired signal frequency,said circuit including reactances of opposite kinds, a first impedanceincluding reactance common to both of said paths and said tuned circuit,and a second impedance common to both of said paths, the ratio of thesecond to the first impedance having a variation as the frequency of thecurrent impressed upon said circuit is varied which is proportional tothe square of the change in frequency.

4. An electrical coupling system comprising a pair of input terminals, apair of output terminals, a path between said input terminals, a pathbetween said output terminals, a circuit tunable to a desired signalfrequency, said circuit including reactances of opposite kinds, a firstimpedance including reactance common to both of said paths and to saidtuned circuit, and a second impedance common to both of said paths, theratio of said second impedance to said first impedance having avariation more rapid than the frequency 50 variation, and saidimpedances also being proportioned to produce substantially zeroresultant transfer through the system at a frequency above the resonantfrequency of said circuit.

5. In a superheterodyne radio receiver, an arrangement for reducingimage frequency interference, which comprises a coupling systemincluding an input circuit, an output circuit, and two individualcoupling means for coupling said circuits, one of said coupling meanscomprising 70 a first impedance means common to said input and outputcircuits, the other coupling means comprising a closed circuit tunableto a desired signal frequency, saidclosed circuit com a second impedancemeans also included in said a capacitive reactance element, one of saidreactance elements being variable to tune said closed circuit, and saidimpedance means being so proportioned that image frequency interferenceproduces in said output circuit equal and opposite voltages across saidfirst impedance means and one of said reactance elements as said closedcircuit is tuned to a desired signal frequency.

6. In a superheterodyne radio receiver, an arrangement for reducingimage frequency interference, which comprises a coupling systemincluding an input circuit, an output circuit, and two individualcoupling means for coupling said circuits, one of said coupling meanscomprising mutual coupling between said input and output circuits, andthe other coupling means comprising a closed circuit tunable to adesired signal frequency, said closed circuit comprising impedance meanscommon to said input circuit,

an inductive reactance element, anda capacitive reactance elementvariable to tune said closed circuit, said coupling and said impedancemeans being so proportioned that the image frequency interferenceproduces equal and opposite voltages across the mutual coupling and aportion of the capacitive reactance of said tuned circuit as the closedcircuit is tuned, whereby image frequency voltages in said outputcircuit are substantially eliminated.

'7. In a superheterodyne radio receiver, an arrangement for reducingimage frequency interference, which comprises a coupling systemincluding an input circuit, an output circuit, and two individualcoupling means for coupling said circuits, one of said coupling meanscomprising mutual inductance between said input and output circuits, andthe other of said coupling means comprising a closed circuit tunable toa desired signal frequency, said closed circuit comprising afixedcapacitive reactance element common to said input circuit, an inductivereactance element, and a capacitive reactance element variable to tunesaid tuned circuit, said mutual inductance and said fixed capacitivereactance element being so proportioned that the image frequencyinterference produces equal and opposite voltages across the inductancein said output circuit and the variable capacitive reactance element ofsaid closed circuit, whereby image frequency voltages in said outputcircuit are substantially eliminated.

8. In a superheterodyne radio receiver, an arrangement for reducingimage frequency interference, which comprises a coupling systemincluding an input circuit, an output circuit, and two coupling meanscoupling said circuits, one of said coupling means comprising mutualinductance between said input and output circuits, the other of saidcoupling means comprising a closed circuit tunable to a desired signalfrequency, said closed circuit comprising an inductance elementinductively related to said input circuit, a fixed capacitance elementalso included in said input circuit, and a capacitance element variableto tune said closed circuit, said first-mentioned mutual inductancebetween said input and output circuits, the mutual inductance betweensaid input and closed circuits and said capacitance element between saidinput and closed circuits being so proportioned that image frequency interference produces equal and opposite voltages across the inductance insaid output circuit external to said closed circuit and the variablecapacitance element of said closed circuit regardsic-rats?" lessof thefrequency to which said-closed circuit is tuned to respond, wherebyimage frequency voltages in saidoutput circuit are substantiallyeliminated.

9. In a superheterodyne radio receiver, means for preventing imagefrequency interference,

which comprises a coupling system including an input circuit, an outputcircuit, and two coupling means between said input and output circuits,one of said coupling means comprising a mutual inductance between saidinput and output circuits and including an inductance one end of whichis at ground potential, and the other coupling means comprising acircuit tunable to a desired signal frequency, said tunable circuitcomprising an inductance inductively related to said input circuit, afixed capacityalso included in said input circuit, and a capacityvariable to tune said circuit, the mutual inductance between said inputand output circuits being so proportioned that the image frequencyvoltage developed across the inductance in said output circuit will beequal and opposite to that developed across the variable condensercommon to said tuned and output circuits regardless of the tuning ofsaid [tuned circuit, whereby image frequency voltages in said outputcircuit are substantially eliminated,

10. In a superheterodyne radio receiver, means for reducing imagefrequency interference, which comprises a coupling system including aninput circuit, an output circuit, and two individual coupling means forcoupling said input and output circuits, one of said coupling meanscomprising a first impedance common to said input and output circuits,the other coupling means comprising a closed circuit tunable to adesired signal frequency; said closed circuit including a secondimpedance also included in said input circuit, an inductance, and acapacitance variable to tune the closed circuit; and said impedancesbeing so proportioned that image frequency interference produces in theoutput circuit a voltage across the first impedance and a voltage acrosssaid capacitance, which voltages are substantially equal and oppositewhen the closed circuit is tuned to the desired signal frequency.

11. In a superheterodyne receiver operative to i select a signal of anydesired frequency in a band of frequencies and subject to interferencefrom another signal of an undesired frequency differing from the desiredfrequency by a constant frequency difi'erence, an arrangement forselectively coupling the antenna to succeeding circuits of said receiverand for reducing said interference comprising a primary circuit adaptedto include said antenna, a secondary circuit coupled to said succeedingcircuits and including a closed circuit tunable over said band, saidclosed circuit including reactances of opposite kinds, and twoindividual coupling means for select a signal of any desired frequencyin a band coupling the ,antenna to succeeding circuits of said receiverand for reducing said interference comprising a primary circuit adaptedto include said antenna, a secondary circuit coupled to said succeedingcircuits, and two individual coupling means for coupling said circuits.one of said means comprising a first impedance means common to saidcircuits, the other of said means comprising a closed circuit tunableover said band, said closed circuit including a second impedance meansalso included in said input circuit and additional reactance means, saidimpedances being so proportioned that image frequency interferenceproduces in said secondary circuit equal and opposite voltages acrosssaid first impedance means and one of said additional reactance meanswhen said closed circuit is tuned to a desired signal frequency, wherebyimage frequency voltages in said secondary circuit are substantiallyeliminated.

13. In combination with a superheterodyne radio receiver adapted toreceive signals throughout a band of frequencies, including a largenumber of signal channels, an antenna circuit broadly tuned to said bandof frequencies whereby all signals having frequencies outside of saidband are attenuated, and a selective circuit coupled between saidantenna circuit and said receiver, said selective circuit comprising aresonant circuit coupled to said antenna circuit and to said receiverandsharply tunable selectively to transmit to said receiver any desiredsignal within said band of frequencies, and an auxiliary transfer meanslikewise coupled to said antenna circuit and to said receiver andoperative to non-selectively transfer signal voltages throughout saidband of frequencies, the coupling of said resonant circuit and saidauxiliary transfer means to said receiver being so proportioned thatthey will deliver equal and opposite voltages to said receiver at anyimage frequency within said band and differing from the frequency towhich said resonant circuit is tuned by a substantially uniformfrequency diflference, whereby said selective coupling circuitsuppresses all image frequency signals within said band.

14. In a superheterodyne radio receiver, the

method of selectively transferring any desired 1 signal current within aband of frequencies including a large number of signal channels, in thepresence of image frequency undesired signals, which comprisespredetermining attenuation of all signals outside of said band,adjustably selectively transferring currents of the desired signalfrequency and incidentally transferring currents of the correspondingundesired image frequency, non-selectively transferring currents of allimage frequencies within said band corresponding to desired signalfrequencies also within said band, and preadiusting the relative valuesand polarity of the first and secondly transferred currents of saidcorresponding image frequency, to balance out the correspondingundesired signal simultaneously with the adjustment to selectivelytransfer the desired signal.

15. In combination with a superheterodyne receiver tunable to signals ofany frequency in a broad band including a large number of signalchannels, an input circuit, and highly selective means and non-selectivemeans each individually coupling said input circuit to the inputterminals of said receiver, said selective means comprising a resonantcircuit tunable over said band, said non-selective means comprisingfixed reactance adjusted to balance out image frequency signalsincidentally coupled to. said receiver by said selective means, and saidinput circuit including resistance and fixed reactance adjusted tobroadly tune said input circuit within said band and thereby attenuateundesired signals of frequencies outside said band coupled to saidreceiver by said non-selective means.

16. In combination with a superheterodyne receiver tunable to signals ofany frequency in a broad band including a large-number of signalchannels, an antenna circuit, and highly selectice means andnon-selective means each individually coupling said antenna circuit tothe first vacum tube of saidreceiver, said selective means comprising aresonant circuit tunable over said band, said non-selective meanscomprising fixed reactance adjusted to balance out image fre-' quencysignals incidentally coupled to said receiver by said selective means,and said antenna circuit including resistance and fixed reactanceadjusted to broadly tune the antenna circuit within said band andthereby attenuate undesired signals of frequencies outside said bandcoupled to said receiver by said non-selective means.

1'7. In combination with a superheterodyne receiver having a permanentlytuned intermediatefrequency amplifier, said receiver being tunable toany frequency in a band greater in width than twice the intermediatefrequency; an input circuit permanently tuned within said band forattenuating undesired signals of frequencies outside said band, aresistance connected in said input circuit for broadening its resonanceto include all of said band, and highly selective means andnon-selective means each individually coupling said input circuit to theinput terminals of said receiver, said selective means comprising aresonant circuit tunable over said band, said non-selective meanscomprising fixed reactance independent of said resistance,adjusted tobalance out image frequency signals within said band incidentallycoupled to said receiver by said selective means, said image frequencydiffering by twice the intermediate frequency from the resonantfrequency of said tunable circuit.

18. In combination with a superheterodyne re- I ceiver having apermanently tuned intermediatefrequency amplifier, said receiver beingtunable to any frequency in a band greater in width than twice theintermediate frequency; an antenna circuit permanently tuned within saidband for attenuating undesired signals of frequencies outside said band,a resistance connected in said antenna circuit for broadening itsresonance to include all of said band, and highly selective means andnon-selective means each individually coupling said antenna circuit tothe first vacuum tube of said receiver, said selective means comprisinga resonant circuit tunable over said band, said non-selective meanscomprising fixed reactance independent of said resistance, adjusted tobalance out image frequency signals within said band incidentallycoupled to said receiver by said selective means, said image frequencydiffering by twice the intermediate frequency from the resonantfrequency of said tunable circuit.

19. A selective coupling network which comprises; a primary circuitincluding connected in series a capacitive antenna, a fixed inductanceand a fixed condenser; a secondary circuit including connected in seriesan adjustable condenser, another fixed inductance and said fixedcondenser; and a third inductance inductively coupled to said firstinductance connected in seties with the outputot said secondary circuit,said of signal channels, said secondary circuit being sharply tunable bysaid adjustable condenser to any frequency in said band, said first two.inductances being 0! thesame order of magnitude, and said fixedcondenser having a capacitance on the order of ten times the capacitanceof said adjustable condensers,

I HAROLD A.

